Field of the Invention
The present invention relates generally to a work vehicle; more specifically the present invention relates to a hybrid work vehicle and a method of controlling the same.
Background Information
Hybrid work vehicles have been proposed in recent years that travel using the drive power from the engine and the drive power from a motor. For instance, Japanese laid-open Patent Application Publication No. 2006-329244 discloses a hybrid work vehicle wherein a hydraulic mechanical transmission (HMT) or an electro-mechanical transmission (EMT) serves as the power transmission device.
The HMT includes a planetary gear mechanism, and a first pump-motor and a second pump-motor connected to the rotational elements in the planetary gear mechanism. The first pump-motor and the second pump-motor function as either a hydraulic motor or a hydraulic pump depending on the travel state of the work vehicle. The HMT continuously varies the rotation speed of the output shafts to thereby change the rotation speed of the pump-motors.
The EMT uses an electric motor in place of the hydraulic motor in the HMT. That is, the EMT includes a first generator-motor and a second generator-motor. The first generator-motor and the second generator-motor function as either an electric motor or an electric generator depending on the travel state of the work vehicle. Similarly to the HMT, the EMT continuously varies the rotation speed of the output shafts to thereby change the rotation speed of the generator-motors.
There is less internal loss in the above-described kinds of hybrid power transmission devices compared to mainstream power transmission devices that include a torque converter and a multi-speed transmission (hereafter referred to as “torque converter transmission”). Consequently, the advantage of a hybrid power transmission is its superior efficiency at transmitting the drive power from the engine to the travel apparatus to obtain traction, and thus its fuel efficiency.
Japanese laid-open Patent Application Publication No. 2006-329244 discloses an example of providing two motors, i.e. a first motor and a second motor in the power transmission device to thereby achieve continuously variable shifting. The invention disclosed in Japanese laid-open Patent Application Publication No. 2006-329244 further includes a mechanism for selectively switching the power transmission mechanism in accordance with the vehicle speed. Switching over to a mechanism that is selectable when the vehicle speed is high is referred to as the Hi mode, while switching over to a mechanism that is selectable when the vehicle speed is low is referred to as Lo mode. FIG. 14 illustrates the relationship between the vehicle speed and the speed of the first motor and the second motor in the invention described in Japanese laid-open Patent Application Publication No. 2006-329244. The speeds of the first motor and the second motor are represented in FIG. 14 by Lm1 and Lm2, respectively. In FIG. 14, the rotation direction of the first motor when the work vehicle 1 travels forward is assumed to be the forward direction; a positive motor speed ratio represents the first motor or the second motor rotating in the forward direction, and a negative motor speed ratio represents the first motor or the second motor rotating in the reverse direction. In the aforementioned work vehicle, the mode in the power transmission device switches at a vehicle speed V1. At and beyond the vehicle speed V1, the power transmission mechanism does not change in Hi mode. Here, the speed of the first motor is zero when the vehicle speed is V2 (e.g., refer to FIG. 2(a) in Japanese laid-open Patent Application Publication No. 2006-329244).